The electron holography, or the electron interference microscopy, is a technique of quantitatively measuring an electromagnetic field in a matter or vacuum by measuring a phase shift of an electron beam caused by a specimen, and specifically a technique in which an electron beam generated in an electron source is splitted into a plurality of electron beams by an electron biprism, a splitted electron beam is made to enter the specimen, and the electron beam having transmitted through the specimen is detected, whereby an interference image is acquired. Such a scanning interference electron microscope is disclosed in, for example, Japanese Patent Application Laid-Open No. 8-45465 and Japanese Patent Application Laid-Open No. 9-134687.
The electron beam holography method is classified in terms of its system into an interference electron microscopy of the scanning transmission electron microscope (STEM; Scanning Transmission Electron Microscope) type and an interference electron microscopy of the transmission electron microscope (TEM; TranSmission Electron Microscope) type. The interference electron microscopy of the STEM type has the following merits as compared with the interference electron microscopy of the TEM type: (1) The STEM type interference electron microscopy can display a phase image on-line and real-time; (2) It can display simultaneously an analytical image, such as detection of a characteristic X-ray etc. generated by scanning illumination of an electron beam, and an interference image; and (3) Since a spatial resolution is determined by a spot size of a focused electron beam, controllability of spatial resolution is excellent; and the like.
An electromagnetic field in the specimen can be estimated by measuring the amount of phase shift of interference fringes by image analysis of a detected interference image, namely the amount of positional shift between positions of constructive interference and of destructive interference. As a technique of measuring the amount of phase shift of interference fringes, for example, there is the method of Leuthner et al. In addition, in the invention disclosed in Japanese Patent Application Laid-Open No. 9-134687, the amount of phase shift is calculated with the method of Leuthner et al. In the Leuthner' method (Th. Leuthner, H. Lichte, and K-H. Herrmann: “STEM-Holography Using the Electron Biprism” Phys. Stat. Sol. A 116, 113. (1989)), a phase image of the specimen is acquired by detecting an electron beam having passed through a grating-type slit with an electron beam intensity detector, and converting an intensity signal of the detected electron beam into phase information. Hereafter, the Leuthner's method will be explained in detail using FIG. 2A and FIG. 2B.
FIGS. 2A and 2B are schematic diagrams each showing a comparative relation among interference fringes of electron beams, a slit, and an electron beam intensity detector. In FIGS. 2A and 2B, the reference numeral 46 denotes a slit and 50 denotes an electron beam intensity detector. The numerals 48 and 49 each denote interference fringes of the electron beams which reach the slit. FIG. 2A corresponds to a case where an aperture of the slit coincides with a position of constructive interference and FIG. 2B corresponds to a case where the aperture of the slit coincides with a position of destructive interference. The vertical axis of the interference fringes 48 and 49 corresponds to the intensity of the electron beams. When performing the method of Leuthner et al., first a direction of the interference fringes and a direction of the slit are set in the same direction. Usually, the apparatus user observes the image of interference fringes by visual inspection, and manually adjusts the direction of the interference fringes obtained, the direction of the slit apertures, and a position of the grating-type slit itself.
When the interference fringes are detected in a state where the direction of the interference fringes agrees with the direction of the slit, the intensity of the detected electron beam varies depending on positions of constructive interference and of destructive interference relative to the slit. In the case of FIG. 2A, the amount of the electron beams passing through the slit 46 becomes a maximum, and in the case of FIG. 2B, the amount of the electron beams passing through the slit 46 becomes a minimum. Therefore, if the amount of the electron beams detected with the detector 50 is normalized using its maximum and minimum, the amount of the detected electron beams could be converted to a cosine of the amount of phase shift. That is, when the amount of the electron beams of the interference fringes passing through the slit 46 assumes a maximum, the phase shift by the specimen is 0□}2π□Λn, and when the amount of the electron beams of the interference fringes passing through the slit 46 assumes a minimum, the phase shift by the specimen is π□}2□Λn. Generally, a direction of the apertures of the slit 46 and a position of the slit 46 are so adjusted that detected constructive interference and destructive interference assume detection intensities of those formed under the condition that there is no specimen or both of the splitted electron beams pass through a vacuum. Therefore, it becomes possible to display an image having phase information of the specimen by displaying the amount of the electron beams having passed through the slit 46 which is normalized to be a value between a maximum and a minimum as a cosine of the amount of phase shift or further converting the value so obtained into the amount of phase shift between zero and π.